U.S. patent application number 10/690399 was filed with the patent office on 2005-04-21 for threshold voltage adjustment for long channel transistors.
Invention is credited to Williford, Ethan.
Application Number | 20050082573 10/690399 |
Document ID | / |
Family ID | 34521640 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050082573 |
Kind Code |
A1 |
Williford, Ethan |
April 21, 2005 |
Threshold voltage adjustment for long channel transistors
Abstract
A threshold voltage adjusted long-channel transistor fabricated
according to short-channel transistor processes is described. The
threshold-adjusted transistor includes a substrate with
spaced-apart source and drain regions formed in the substrate and a
channel region defined between the source and drain regions. A
layer of gate oxide is formed over at least a part of the channel
region with a gate formed over the gate oxide. The gate further
includes at least one implant aperture formed therein with the
channel region of the substrate further including an implanted
region within the channel between the source and drain regions.
Methods for forming the threshold voltage adjusted transistor are
also disclosed.
Inventors: |
Williford, Ethan; (Boise,
ID) |
Correspondence
Address: |
TRASK BRITT
P.O. BOX 2550
SALT LAKE CITY
UT
84110
US
|
Family ID: |
34521640 |
Appl. No.: |
10/690399 |
Filed: |
October 20, 2003 |
Current U.S.
Class: |
257/202 ;
257/E21.421; 257/E21.618; 257/E21.633; 257/E29.264 |
Current CPC
Class: |
H01L 29/7831 20130101;
H01L 21/823807 20130101; H01L 29/6659 20130101; H01L 29/7816
20130101; H01L 29/66681 20130101; H01L 29/66484 20130101; H01L
21/823412 20130101; H01L 21/26586 20130101; H01L 29/66537 20130101;
H01L 29/6653 20130101 |
Class at
Publication: |
257/202 |
International
Class: |
H01L 027/10 |
Claims
1. A threshold-adjusted transistor, comprising: a substrate
including: spaced-apart source and drain regions formed in the
substrate; and a channel region defined between the source and
drain regions; a layer of gate oxide formed over at least a part of
the channel region; and a gate formed over the layer of gate oxide,
the gate further having at least one implant aperture formed
therein, the channel region of the substrate further including a
channel internal implanted region between the source and drain
regions.
2. The transistor of claim 1, the substrate further comprising at
least one lightly-doped structure located between at least one of
the source and drain region and the channel region.
3. The transistor of claim 2, further comprising a double-diffused
structure at least partially surrounding each of the channel
internal implanted region and the at least one lightly-doped
structure.
4. The transistor of claim 3, wherein the double-diffused structure
is implanted at a diagonal angle to the gate and through the at
least one implant aperture of the gate.
5. The transistor of claim 1, the substrate further comprising an
enhancement region located within at least a portion of the channel
region.
6. The transistor of claim 2, wherein the channel internal
implanted region and the at least one lightly-doped structure are
formed according to the same fabrication process.
7. The transistor of claim 2, wherein the at least one
lightly-doped structure is a lightly-doped drain (LDD) structure
arranged between one of the drain and source regions and the
channel region.
8. The transistor of claim 1, wherein the at least one implant
aperture comprises a plurality of implant apertures arranged in a
checkerboard configuration along the gate.
9. The transistor of claim 1, wherein the at least one implant
aperture comprises a plurality of implant apertures arranged in a
two dimensional array configuration along the gate.
10. A method for forming a transistor, comprising: forming a gate
having source and drain ends and insulated from a substrate, the
gate including at least one aperture extending therethrough to the
substrate; forming doped regions in the substrate adjacent to the
source and drain ends as separated by a channel region and in the
substrate below the at least one aperture of the gate; and forming
source and drain regions in the substrate adjacent to the source
and drain ends of the gate.
11. The method of claim 10, further comprising forming at least one
lightly-doped structure located between at least one of the source
and drain regions and the channel region.
12. The method of claim 11, further comprising forming a
double-diffused structure at least partially surrounding each of
the doped regions below the at least one aperture of the gate and
the at least one lightly-doped structure.
13. The method of claim 12, further comprising implanting the
double-diffused structure at a diagonal angle to the gate and
through the at least one aperture of the gate.
14. The method of claim 10, further comprising forming an
enhancement region in the substrate located within at least a
portion of the channel region.
15. The method of claim 10, further comprising forming the doped
regions according to a single fabrication process.
16. The method of claim 11, further including forming the at least
one lightly-doped structure as a lightly-doped drain (LDD)
structure arranged between one of the source and drain regions and
the channel region.
17. The method of claim 10, wherein the at least one aperture
comprises a plurality of apertures arranged in a checkerboard
configuration along the gate.
18. The method of claim 10, wherein the at least one aperture
comprises a plurality of apertures arranged in a two dimensional
array configuration along the gate.
19. In a fabrication process optimized for short-channel
transistors, a method for adjusting a threshold voltage of a
long-channel transistor, comprising: forming at least one aperture
in a gate on a substrate; implanting a first dopant through the at
least one aperture into a channel region of the substrate; and
annealing the first dopant in the channel region.
20. The method of claim 19, wherein the implanting and annealing
the first dopant utilizes implanting and annealing processes for
forming lightly-doped drains of the short-channel transistors.
21. The method of claim 19, further comprising implanting a second
dopant into the at least one aperture and annealing the second
dopant to form a double-diffused structure at least partially
surrounding the first dopant.
22. The method of claim 21, wherein the implanting of the second
dopant occurs at an angle off-vertical through the at least one
aperture of the gate.
23. The method of claim 19, wherein forming comprises arranging a
plurality of apertures along the length of the gate.
24. The method of claim 23, wherein the plurality of apertures is
arranged in a checkerboard configuration along the gate.
25. The method of claim 23, wherein the plurality of apertures is
arranged in a two dimensional array configuration along the
gate.
26. A method for manufacturing an MOS structure on a semiconductor
substrate, comprising: forming a gate oxide layer over the
semiconductor substrate; forming a polysilicon layer over the gate
oxide layer; patterning and etching a first mask layer to form a
gate having an aperture between source and drain ends in the
polysilicon layer; forming implant regions in the substrate
adjacent to the source and drain ends and within the aperture of
the gate; and forming source and drain regions in the substrate
adjacent to the source and drain ends of the polysilicon layer.
27. The method of claim 26, wherein forming the implant regions
comprises forming lightly-doped drain structures in the
substrate.
28. The method of claim 27, wherein forming the implant regions
further comprises forming double-diffused structures at least
partially surrounding the lightly-doped drain structures.
29. The method of claim 28, further comprising implanting the
double-diffused structures at a diagonal angle to the gate and
through the aperture of the gate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to semiconductor devices and,
more particularly, to threshold voltage adjustment of long channel
MOS transistors.
[0003] 2. State of the Art
[0004] Metal-Oxide-Semiconductor (MOS) is the primary technology
for large-scale integrated semiconductor circuits. In Complementary
MOS (CMOS) architectures, these semiconductor circuits combine two
types of MOS devices, namely p-channel MOS (PMOS) devices and
n-channel MOS (NMOS) devices, on the same integrated circuit. A MOS
transistor is a four-terminal device which controls the current
that flows between two of the terminals by activating and
deactivating the voltage which is applied to the third or fourth
terminal. FIG. 1 shows a cross-sectional diagram of a conventional
n-channel MOS transistor 10. As shown in FIG. 1, transistor 10
includes spaced-apart n+source and drain regions 12 and 14 which
are formed in a p-type substrate 16, and a channel region 18 which
is defined between source and drain regions 12 and 14. Source and
drain regions 12 and 14, in turn, represent the first two terminals
of the device while substrate 16 represents the third terminal.
[0005] Transistor 10 also includes a layer, illustrated as gate
oxide 20, which is formed over channel region 18, and a gate 22
which is formed over gate oxide layer 20. Gate 22 represents the
fourth terminal of the device. During operation of the transistor
10, electrons flow from source region 12 to drain region 14 when an
electric field is established between source and drain regions 12
and 14. Furthermore, the drain-to-substrate junction is reverse
biased when a gate voltage equal to or greater than the threshold
voltage of transistor 10 is applied to gate 22. These conditions
can be met, for example, when ground is applied to substrate 16 and
source region 12, and one volt, for example, is applied to drain
region 14.
[0006] A gate voltage applied to gate 22 attracts electrons to the
surface adjacent to gate oxide 20 of substrate 10 in channel region
16. When a minimum number of electrons has been attracted to the
surface of substrate 16 in channel region 18, the electrons form a
channel which allows the electrons in source region 12 to flow to
drain region 14 under the influence of the electric field. The
threshold voltage is defined as the minimum gate voltage that must
be applied to gate 22 to attract the minimum number of electrons to
the surface of substrate 16 to form an electrically conductive
inversion region in the channel 18.
[0007] The threshold voltage of transistor 10 may be altered or
adjusted by implanting the surface of substrate 16 in channel
region 18 with, for example, a p-type dopant which, in turn,
decreases the number of electrons that can be accumulated at the
surface in the channel region 18. Since fewer electrons are
available, a higher gate voltage is needed to attract the minimum
number of electrons that are required to form an inversion layer in
the channel. A threshold voltage adjustment implant is commonly
referred to as an "enhancement" implant.
[0008] MOS transistors are formed using photolithographic processes
according to design rules corresponding to a particular process.
The design rules specify, among other things, the minimum length of
the channel region. To gain performance advantages and as
processing technology advancements have been achieved, the channel
length between the source and drain has generally shortened.
Furthermore, to minimize the silicon area consumed by a MOS
circuit, a typical integrated circuit design is largely implemented
with transistors that have the minimum channel length. Since the
circuit is largely implemented with transistors that have the
minimum channel length, the fabrication process, for example the
enhancement implant, is commonly optimized to adjust the threshold
voltages of the transistors which have the minimum channel length.
While performance improvement is generally a paramount objective
for MOS circuit design, it is common for circuits in addition to
utilizing transistors having minimum channel length to also require
transistors which have channel lengths that are longer than the
minimum. For those transistors with a longer channel length, a
lower threshold voltage is realized when the threshold voltage is
optimized for a shorter-channel transistor through the use of a
single enhancement implant.
[0009] FIG. 2 is a graph that generally plots the threshold
voltages as a function of channel length. As shown in FIG. 2, when
the threshold voltage is optimized for an arbitrary fabricatable
channel length x, the threshold voltage of a transistor decreases
as the channel length of the transistor increases. Furthermore, the
reduced threshold voltages of the longer channel devices lead to
increased leakage currents which, in turn, are particularly
undesirable in circuits which are utilized in battery-operated
devices.
[0010] One prior solution to this problem is to utilize multiple
threshold voltage adjusting enhancement implants. In a first step,
dopants are implanted into the surface of the substrate to adjust
the threshold voltages of the short channel transistors while the
long channel transistors are protected from the implant. In a
second step, dopants are implanted into the surface of the
substrate to adjust the threshold voltages of the long or longer
channel transistors while the short-channel transistors are
protected from the implant. By utilizing, for example, two implant
steps, the dopant concentration for the short and long channel
lengths can be separately optimized.
[0011] One shortcoming to the multiple-threshold adjusting implant
approach, however, is that utilizing separate implant steps
requires separate masks which, in turn, increases the cost of
fabricating the circuit. Thus, there is a need for adjusting the
threshold voltage of long-channel MOS transistors to a higher
threshold voltage when the long-channel transistor is fabricated
with a single threshold-voltage implant step that is optimized to
set the threshold voltage of a short-channel transistor.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention, in exemplary embodiments, is directed
to threshold voltage adjustments for long channel transistors
fabricated using processes optimized for short channel transistors.
In one embodiment of the present invention, a threshold-adjusted
transistor includes a substrate with spaced-apart source and drain
regions formed in the substrate and a channel region defined
between the source and drain regions. A layer of gate oxide is
formed over at least a part of the channel region with a gate
formed over the gate oxide. The gate further includes at least one
implant aperture formed therein with the channel region of the
substrate further including an implanted region within the channel
between the source and drain regions.
[0013] In another embodiment of the present invention, a method is
provided for forming a transistor. A gate is formed having source
and drain ends and insulated from a substrate with the gate
including at least one aperture extending therethrough to the
substrate. Doped regions are formed in the substrate adjacent to
the drain and source ends as separated by a channel region and also
one or more doped regions are formed in the substrate below the
aperture of the gate. Source and drain regions are then formed in
the substrate adjacent to the source and drain ends of the
gate.
[0014] In yet another embodiment of the present invention, a method
is provided for adjusting a threshold voltage of a long channel
transistor in a fabrication process optimized for short-channel
transistors. At least one aperture is formed in a gate on a
substrate with a first dopant implanted through the at least one
aperture into a channel region of the substrate. The first
implanted dopant is annealed into the channel of the long channel
transistor.
[0015] In yet a further embodiment of the present invention, a
method for manufacturing a MOS structure on a semiconductor
substrate is provided. A gate oxide layer is formed over the
semiconductor substrate with a polysilicon layer being also formed
over the gate oxide layer. A first mask layer is formed and
patterned followed by etching to form a gate. The gate includes an
aperture between the source and drain ends of the polysilicon
layer. Implant regions are formed in the substrate adjacent to the
drain and source ends of the gate. Also, at least one implant
region is formed in the substrate through the aperture of the gate.
Source and drain regions are formed in the substrate adjacent to
the source and drain ends of the gate.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0016] In the drawings, which illustrate what is currently
considered to be the best mode for carrying out the invention:
[0017] FIG. 1 illustrates the elements of a semiconductor
transistor, in accordance with the prior art;
[0018] FIG. 2 graphically illustrates the relationship between a
transistor threshold voltage and the channel length of a
transistor, in accordance with the prior art;
[0019] FIG. 3 is a perspective view of a transistor having a gate
with internal implant apertures, in accordance with an embodiment
of the present invention;
[0020] FIG. 4 is a cross-sectional view of a transistor having a
gate with internal implant apertures, in accordance with an
embodiment of the present invention;
[0021] FIGS. 5A-5F illustrate processing steps for forming a
transistor including implant regions throughout the channel, in
accordance with an embodiment of the present invention;
[0022] FIG. 6 illustrates an arrangement of implant windows along a
gate of a transistor, in accordance with an embodiment of the
present invention;
[0023] FIG. 7 illustrates an arrangement of implant windows along a
gate of a transistor, in accordance with another embodiment of the
present invention; and
[0024] FIG. 8 illustrates angular relationships of an implant
aperture, in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] While short channel transistors are susceptible to decreased
threshold voltages and therefore utilize threshold voltage
adjusting enhancement implants for adjusting the threshold voltage,
short channel transistors are also susceptible to so-called "hot
carrier effects." Generally, as the channel length is shortened,
the maximum electric field E.sub.m becomes more isolated near the
drain side of the channel, causing a saturated condition that
increases the maximum energy on the drain side of the MOS device.
The high energy causes electrons in the channel region to become
"hot". An electron generally becomes hot in the vicinity of the
drain edge of the channel where the energy arises. Hot electrons
can degrade device performance and cause breakdown of the device.
Moreover, the hot electrons can overcome the potential energy
barrier between the silicon substrate and the silicon dioxide layer
overlying the substrate, which causes hot electrons to be injected
into the gate oxide.
[0026] Problems arising from hot carrier injections into the gate
oxide include generation of a gate current and generation of a
positive trapped charge which can permanently increase the
threshold voltage of the MOS device. These problems are manifested
as an undesirable decrease in saturation current, decrease of the
transconductance and a continual reduction in device performance
caused by trapped charge accumulation. Thus, hot carrier effects
cause unacceptable performance degradation in MOS devices built
with conventional drain structures when channel lengths are
short.
[0027] Reducing the maximum electric field E.sub.m in the drain
side of the channel is a popular way to control the hot carrier
injections. A common approach to reducing E.sub.m is to minimize
the abruptness in voltage changes near the drain side of the
channel. Disbursing abrupt voltage changes reduces E.sub.m strength
and the harmful hot carrier effects resulting therefrom. Reducing
E.sub.m occurs by replacing an abrupt drain doping profile with a
more gradually varying doping profile. A more gradual doping
profile distributes E.sub.m along a larger lateral distance so that
the voltage drop is shared by the channel and the drain. Absent a
gradual doping profile, an abrupt junction can exist where almost
all of the voltage drop occurs across the channel. The smoother or
more gradual the doping profile, the smaller E.sub.m becomes
resulting in reduced hot carrier injections.
[0028] One approach for minimizing the effects associated with hot
carrier injections includes forming a modified drain structure,
such as lightly doped drain (LDD) structure. LDD structures provide
a doping gradient at the drain side of the channel that lead to the
reduction in E.sub.m. The LDD structures act as parasitic resistors
to absorb some of the energy into the drain and thus reduce maximum
energy in the channel region. This reduction in energy reduces the
formation of hot electrons. In most typical LDD structures of MOS
devices, sources/drains are formed by two implants with dopants,
one implant self-aligned to the polysilicon gate to form shallow
source/drain extension junctions that are lightly doped
source/drain regions and a second implant with a heavier dose to
form the actual deep source/drain junctions.
[0029] In addition to the concern over hot carrier injection in
short channels, a condition known as "punch-through" is also of
concern. To further protect the transistor from punch-through
conditions, a double diffusion (DD) process may further surround
the LDDs. The DD process implants one or more dopants into the same
region followed by a high temperature annealing step, in which the
one or more dopants diffuse simultaneously, and form a structure
called a double-diffused (DD) region, also commonly called a
double-diffused drain (DDD). In an exemplary DD process, a medium
phosphorus dose and a heavy arsenic dose may be implanted; but in
both case a p-type Boron halo implant is put in to surround the
n-type LDD implant to protect against "punch-through." The
faster-diffusing phosphorus is driven farther under the gate edge
than the arsenic, creating a less abrupt concentration gradient for
the drain.
[0030] When transistor channel lengths are several times longer
than the diameter of the LDD and DD regions, then the threshold
voltage adjusting or enhancement implant exhibits the dominate
effect over the other adjacent implants. However, as fabrication
processes improve, transistor channel lengths generally decrease.
In short-channel transistors that include one or both of an LDD or
DD region, the LDD or DD implant diffusions with their larger
dopant concentrations more greatly influence the threshold voltage
adjustment than does the enhancement implant. Therefore, processes
that include both short and long channels would necessarily exhibit
differing threshold voltages since the threshold voltage of
short-channel transistors is more greatly influenced by the LDD and
DD implants while the threshold voltage of the long-channel
transistors is more greatly influenced by the threshold voltage
adjusting enhancement implant.
[0031] FIG. 3 is a perspective view of a transistor incorporating a
channel implant process, in accordance with an embodiment of the
present invention. A transistor 30 is formed upon and within a
substrate 42 and generally includes a gate 32 formed upon a gate
oxide 28 according to processes known by those of ordinary skill in
the art. Transistor 30 further includes a drain region 48 and a
respective source region 50 generally formed within substrate 42.
In accordance with the channel implant process of an embodiment of
the present invention, gate 32 further includes one or more
apertures 34, 36 formed within the general body of gate 32.
Apertures 34, 36 provide internal implant windows into the channel
region 52 located generally below gate 32. The quantity of
apertures 34, 36 is a function of the dimensions of the channel,
namely channel length 44 and channel width 46. As viewed in a top
view, apertures 34, 36 may be in the shape of a square, rectangle,
circle, polygon, or any other uniform or non-uniform shape.
However, a square aperture is shown in the drawing figures, and
used as an example hereinafter. Additionally, the aperture, when
viewed from the top, may be entirely or partially enclosed or
surrounded by gate 32. For example, the aperture may be located on
the edge of the gate 32 and not entirely enclosed with the gate
material resulting in a notch, grove, keyhole, or the like.
However, it is currently preferred that the aperture be enclosed
within gate 32. The aperture may be formed by conventional mask and
etch procedures either before or after the gate 34 is formed.
[0032] FIG. 4 illustrates a cross-sectional view of transistor 30
formed according to the channel implant process, in accordance with
an embodiment of the present invention. Transistor 30 further
includes a gate 32 illustrated as including cross-sectional gate
portions 32', 32" and 32'" which are separated by apertures 34, 36.
Gate 32 is separated from substrate 42 by an insulating gate oxide
28 created in a channel region 52 on the side of substrate 42
adjacent to gate oxide 28.
[0033] Transistor 30 also includes drain region 48 and source
region 50 separated by the channel region 52. Drain region 48
includes a main portion and an adjacently located inward lightly
doped extension illustrated as LDD structure 54. LDD structure 54
may extend slightly under gate portion 32'". The source region 50
of transistor 30 is formed simultaneously with the drain region 48
and is typically configured in a like manner. The source region 50
includes a main source portion and a more lightly doped extension
illustrated as LDD structure 56. Similarly, through one or more
apertures 34, 36, channel internal implanted regions such as
additional LDD structures 58, 60 are also formed. The purpose of
the first implant is to form LDD structures at the edge near the
channel and to provide internal implanting within the channel. In
an LDD structure, almost the entire voltage drop occurs across the
lightly doped drain region.
[0034] Additional more heavily doped implants such as double
diffused (DD) implants may also be formed about drain region 48 and
source region 50. DD implants are generally illustrated as DD
structures 62, 64 and are typically more heavily doped than the
original enhancement implants generally illustrated as enhancement
implant 66. In accordance with an embodiment of the present
invention, DD implants are also performed through apertures 34, 36
to form DD structures 68, 70 located within the internal channel
region 52 of transistor 30.
[0035] By way of example and not limitation, the relative doping
densities as described herein include enhancement implants on the
order of E.sup.12 and LDD implants on the order of E.sup.12.
Furthermore, DD implants are implanted at, for example, densities
on the order of E.sup.13 while the source and drain regions are
implanted at levels on the order of E.sup.15. Therefore, it is
evident that for short channel transistors where the LDD and DD
structures occupy a significant portion of the channel length, the
LDD and DD implants more greatly influence the threshold voltage of
the transistor than a threshold voltage adjusting enhancement
implant. Embodiments of the present invention enable implanting
through the use of apertures distributed about the internal
arrangement of the longer gate structures dominating doping
densities into the longer channels consistent with the dominant
doping densities implanted in shorter channel devices.
[0036] To form the source and drain regions, spacers 114, 116 (FIG.
5E) are formed around the gate. With the shallow drain extension
junctions protected by the spacers, a second implant with heavier
dose is self-aligned to the oxide spacers around the gate to form
deep source/drain junctions illustrated as source/drain regions 50,
48. Generally, a rapid thermal annealing occurs (RTA) to enhance
the diffusion of the dopants implanted in the deep source/drain
junctions. The source/drain implant with heavier doses form low
resistance deep drain junctions, which are coupled to the LDD
structures. Since the source/drain implant is spaced from the
channel by the spacers, the resulting drain junction adjacent to
the light doped drain region can be made deeper without impacting
device operation. The increase junction depth lowers the sheet
resistance and the contact resistance of the drain.
[0037] In most typical LDD structures for CMOS devices,
sources/drains are formed by four implants with dopants, each
implant requiring a masking step. The four masking steps are: a
first mask (a P-LDD mask) to form the P-LDD structures, a second
mask (an N-LDD mask) to form the N-LDD structures, a third mask (a
P+ S/D mask) to form the p-type doped, deep source/drain junctions,
and a fourth mask (an N+ S/D mask) to form the N-type doped, deep
source/drain junctions. Each masking step typically includes the
sequential steps of preparing the semiconductor substrate, applying
a photoresist material, soft-baking, patterning and etching the
photoresist to form the respective mask, hard-baking, implanting a
desired dose of a dopant with the required conductivity type,
stripping the photoresist, and then cleaning of the substrate.
[0038] FIGS. 5A-5F (hereinafter collectively "FIG. 5") illustrate a
cross-sectional view of a portion of the silicon wafer during
various processing steps to create the transistor structure of FIG.
4, according to an embodiment of the present invention. While the
present illustration depicts a p-type substrate forming an
n-channel MOS transistor, a p-channel transistor may also be
implemented according to the processes described herein as modified
by processes known by those of ordinary skill in the art. Referring
to FIG. 5A, a substrate 42 has applied thereon a masking layer 82
which is applied and patterned according to processes known by
those of ordinary skill in the art. The patterning process creates
an opening 84 which corresponds to the channel region 52 of a
developing transistor. An enhancement implant 86 creates an
enhancement region 88 which typically facilitates threshold voltage
adjustment in a transistor device. By way of example, the threshold
voltage of an n-channel transistor would be adjusted by introducing
a p-type dopant, typically boron, into at least a portion of the
transistor channel.
[0039] FIG. 5B illustrates the process for forming an LDD
structure, in accordance with an embodiment of the present
invention. In this process, a gate insulating oxide 90 is formed
over the previously defined channel region 52 with a gate layer 92,
such as a polysilicon layer, deposited over gate oxide layer 90.
Furthermore, gate 32 has formed therein one or more apertures 34,
36 for facilitating implantation within the outer periphery of gate
32. Using gate 32 as a mask, impurity ions, for example n-type
impurity ions are generally vertically implanted as LDD implant 94
into substrate 42 in a low concentration, thereby forming LDD
structures 96, 98, 100, 102.
[0040] FIG. 5C illustrates a double diffusion (DD) implant 104 into
substrate 42 for forming punch-through inhibitors. For the
n-channel example of the present embodiment, a p-type dopant is
angle-implanted as an ion implantation with portions of the p-type
implant penetrating laterally under gate portions 32', 32", 32'"
due to the angular implant process. In a preferred implantation
process, substrate 42 is rotated in preferably four separate
rotations for facilitating the angular implantation about the
generally rectangular profile of gate 32. The angular DD implant
104 results in implant regions 106, 108, 110, and 112.
[0041] In an annealing process as depicted in FIG. 5D, the various
implantation ions of both the LDD implants 94 and DD implants 104
result in further penetration of the implant ions into substrate
42. As illustrated, the LDD or n-regions 96-102 as well as the DD
implant regions 106-112 migrate under the channel region 52 located
under the gate 32. Migration of the higher dopant concentrations of
the LDD implants and the DD implants minimizes the impact of the
enhancement implant of FIG. 5A and causes the threshold voltage to
change according to the additional dopant concentrations.
[0042] In FIG. 5E, spacers 114, 116 as well as implant shields 118,
120 protect the underlying structures from the high concentration
source/drain implant 122 while the source region 50 and drain
region 48 are implanted. As illustrated in FIG. 5F, spacers 114,
116 and implant shields 118, 120 are removed to complete the
formation of transistor 30. As illustrated, transistor 30 is
comprised of a gate 32, drain and source regions 48, 50 as well as
drain and source specific LDD structures 54, 56 and drain and
source specific DD structures 62, 64. Transistor 30 further
comprises one or more internal LDD structures 58, 60 as well as one
or more internal DD structures or regions 68, 70. The formation of
internal structures through the use of implants throughout the
length of the channel more consistently aligns the threshold
voltage of a long channel transistor with the threshold voltage of
a short channel transistor and thereby reduces the disparate
threshold voltages between respective long and short channel
transistors while eliminating independent threshold voltage
enhancement implants for long and short channel transistors.
[0043] FIGS. 6 and 7 illustrate grid arrangements of apertures,
according to specific embodiments of the present invention. In FIG.
6, the formation of a gate 32 includes an exemplary pattern of
apertures 136 for the gate 134 shown in a "checkerboard" pattern
between a source region 138 and a drain region 140. In addition, an
exemplary method for calculating the width of aperture 136 with
respect to gate 134 height and implantation angle (b) is shown in
FIG. 8. One current exemplary embodiment includes a gate length of
about 0.1 to 4 microns. FIG. 7 illustrates the formation of
apertures in a gate, in accordance with another embodiment of the
present invention. A gate 142 includes an exemplary two-dimensional
array of apertures 144 located between a source region 146 and a
drain region 148. Further geometries and aperture shapes are also
contemplated within the scope of the present invention.
[0044] FIG. 8 illustrates an exemplary aperture, in accordance with
an embodiment of the present invention. The dimensions (A) 150 of
an aperture 152 may be determined by the gate height, desired ion
implantation angle (b) 154, and process or manufacturing
capabilities. For example, as shown in FIG. 8, if the gate height
is 1200 .ANG. and the desired implantation angle (b) is 65.degree.,
the aperture width (A) 150 would be (1200 .ANG./tan 65.degree.) or
560 .ANG.. With current process or manufacturing capabilities, the
aperture 152 dimensions are currently preferred to be greater than
0.02 by 0.02 microns. However, the aperture 152 dimension (A)
should be small enough to allow the fringe effects from the gate
field to electrically invert the charge of the area under the
aperture when a voltage is applied to the gate.
[0045] The angle (b) 154 shown in FIG. 8 is the angle for driving
the dopants into the under layer in the channel region 52 (FIG.
5C). As an example, for a square aperture, this implantation will
take place through the device apertures and gate edge and then the
device or implantation equipment will be rotated 90.degree. and
then the implantation will take place again. This process will
allow for the dopant to be implanted into the channel region 52 of
the device on all four sides of a square aperture once a full
revolution has been completed. The device or implantation device
may turn or rotate more or less times and at a greater or lesser
angle to implant the dopant into the channel region 52 depending on
the shape of the aperture to get a uniform diffusion under the
aperture. Implanting the dopant at the angle (b) 154 will create
"fringe" effect regions in the channel region 52 of the device. The
angle (a) 156 of the implant at the apertures is currently
preferred to be 0 to 25 degrees. The angle is determined by how far
the dopant is to be driven under the gate. For example, a small
angle (a) 156 will not drive the dopant as far under the gate as a
large angle (a) 156 implant. As a result of a smaller angle (a)
156, the fringing of the implant will be reduced in the channel
region 52. An exemplary ion implant is phosphorus (Ph) to provide a
negative (n-) doping of the channel region 52.
[0046] Embodiments of the present invention utilize the implant
process for forming one or more of an LDD or a DD region in a
short-channel transistor for "enhancing" the concentration of
dopants in the channel of a long-channel transistor. Therefore,
both the short-channel and long-channel transistors exhibit similar
threshold voltages as adjusted by the LDD and/or DD implanting
processes internal to the channel.
[0047] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and have been described in
detail herein. However, it should be understood that the invention
is not intended to be limited to the particular forms disclosed.
Rather, the invention includes all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the following appended claims.
* * * * *